Transponder with AGC regulator

ABSTRACT

For the digital data communication between a vehicle (transponder) and a beacon, an automatic gain control wherein the path attenuation between the transponder and the brake increases disproportionately with the distance. A two-stage automatic gain control is included for the gain control. After the first amplifier stage, the output signal is measured and supplied, together with a setpoint value contained in a setpoint memory, to a control amplifier. The output signal of the control amplifier is used both for automatically controlling the first amplifier stage, as well as the second amplifier stage. The automatic control makes it possible to cover only one single lane when automatically debiting highway tolls for a motor vehicle, for example. Influences caused by other motor vehicles on a nearby lane are eliminated.

FIELD OF THE INVENTION

The present invention relates to a transponder, preferably for thebidirectional transmission of data to a fixed station designed as abeacon.

BACKGROUND INFORMATION

A transponder for the bidirectional transmission of data between avehicle's OBU (on-board unit) and a fixed station (beacon), which worksin accordance with the semipassive transponder method, is alreadydisclosed by the publication Proceedings of the DRIVE Conference,February 4-6, 1991, "Advanced Telematics in Road Transport" (pp.248-268). However, the problem occurs that the range of the beacon'sreception coverage, for example, is not able to be restricted preciselyenough to one single lane (on the road), so that in an automatic tollticketing (collection) application, it can happen that the OBU of avehicle driving in a nearby lane so greatly disturbs the communicationof the vehicle driving under the antenna that the tolls are not properlydebited.

SUMMARY OF THE INVENTION

In contrast, the advantage of the transponder according to the presentinvention is that through the automatic feedback control of the signalthat is retransmitted (sent back) to the beacon, the attenuation of thesignal is so conceived that only a strictly limited range is providedfor the signal level arriving at the beacon. The attenuation is socontrolled in this case that the attenuation of the signal both from thebeacon to the vehicle, as well as in the opposite direction iscompensated and, as a result, possible interference from nearby laneareas is suppressed. OBUs situated in the nearby lane are limitedaccordingly out of the corresponding antenna because of their higheruseful signal level and can no longer interfere.

It is also advantageous according to the present invention that thedownlink signal is controlled by a first automatically controllableamplifier stage (automatic gain control, AGC). There follows, as aresult, a first measurement of the level received. For example, thelevel can vary due to a dirty (window) pane or because of otherinfluences. The level is measured to find a controlled variable withreference to a setpoint value and to use it as a basis for the automatic(feedback) control.

Another advantage of an embodiment according to the present invention isthat the variable found in the first measurement is fed for controlpurposes to another amplifier stage, which amplifies or attenuates theUPLINK signal with the same factor and the retransmits it to the beacon.

In exemplary applications, the advantageous results according to thepresent invention are attained when allocating the reception rangecoverage of a beacon to one lane that only one vehicle is covered by thebeacon at a time. As a result, when the transponder is mounted on amotor vehicle, one single vehicle can be covered. This is especiallyadvantageous for the automatic debiting of highway tolls or for checkingif vehicles have authorized access.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustration of the transmit mode in DOWNLINKaccording to the present invention.

FIG. 2 shows a schematic illustration of the transmit mode in UPLINKaccording to the present invention.

FIG. 3 illustrates a block diagram of an embodiment according to thepresent invention.

FIG. 4 illustrates an example of transition of signals in multiple laneson a road according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a motor vehicle 2, which is equipped with an (OBU)"on-board unit" or digital communication with a beacon 4. The OBU 1works as a transponder 3 and receives the amplitude-modulated signalsASK of the beacon 4 in DOWNLINK.

The transponder 3, as well as the beacon 4 are known from the previouslymentioned publication and, therefore, require no further clarification.The transponder 3 has a receiving antenna for receiving data signals anda transmitting antenna 1 for transmitting data signals. When microwavesare used, the antennas 1 constitute a part of the transponder.

FIG. 2 depicts the digital communication in UPLINK, the transponder 3(as shown in FIG. 1) initially generating its line voltage from theunmodulated signal of the beacon 4 and then transferring its data to betransmitted in FSK modulation (frequency shift keying) to the beacon 4.When working with the passive, per se known transponder method, thedisadvantage results that the UPLINK transmit energy is dependent uponthe attenuation on the transmission path. This is elucidated on thebasis of the example of FIG. 4. In FIG. 4, two lanes 20, 21 are shown,on which a first motor vehicle 2 or a second motor vehicle 2 is beingdriven. A beacon 4 with antennas 22, 23 is mounted on a gantry (overheadsign) spanning the lanes. The antennas are so arranged that they areable to cover the indicated lane area 20 or 21. Now, it is so that theantenna 22 also covers the lane area 21 and, conversely, the antenna 23covers the lane area 20, although with very low power (aerial) gain. Inaddition, the problem arises that the path attenuation between thebeacon 4 and the transponder 3 (as shown in FIG. 1) is dependent uponvarious factors. An example is when the first motor vehicle is situatedclose to the antenna 22, and the second motor vehicle is further awayfrom the antenna 23. It can happen that, as a truck (or a commercialvehicle), the first motor vehicle has a high type of construction andthat, as a passenger vehicle, the second motor vehicle has a low type ofconstruction. Also in this example, the first motor vehicle has a cleanwindowpane, behind which the transponder 3 is situated, while the secondmotor vehicle has dirty windowpanes. As a result of these factors, thepath attenuation between the beacon 4 and the first vehicle 2 isconsiderably lower than in the case of the second vehicle. Because ofthe low path attenuation of the first motor vehicle 2, the antenna 3 isalso able to receive its signals.

To eliminate these shortcomings, in accordance with FIG. 3, the presentinvention proposes introducing means for a gain control 12 (automaticgain control, AGC) between the receiving and transmitting antenna 1. Afirst improvement in the feedback control is already achieved by keepingthe output signal from the transponder 3 (shown in FIG. 1) constant withthe aid of a first, automatic gain control 7. This signal that is keptconstant and has a predetermined output level can then be retransmittedto the beacon 4 (as shown in FIGS. 1, 2 and 4).

A still further improved control of the output signal according to thepresent invention is obtained, as illustrated in FIG. 3, by connecting asecond amplifier stage 8 in series with the first amplifier stage 7 andby connecting its output to the transmitting antenna 1. For thispurpose, the amplifier control 12 has a detector 5, which detects theamplitude of the first amplifier stage 7. This signal is transmitted toa control amplifier 6, which extracts its reference value from asetpoint generator 11. The setpoint generator 11 is designed as a memorydevice. Now, on the basis of the predetermined setpoint values and thedetector signal, the control amplifier 6 automatically controls itsoutput gain and transmits a corresponding controlled value intocorresponding control inputs 9, 10 of the first and second amplifierstage.

The advantage of the feedback control method according to the presentinvention consists in that, for example, a gain measurement is onlyrequired at the first amplification stage and can be easily carried outwith the detector 5. An appropriate diode circuit is suited as adetector. Commercially available semiconductor controllers can be usedas control amplifiers 6. However, the control characteristic of thefirst and second amplifier stage 7, 8 should conform to the greatestextent possible. The effect of this double gain control is that theoutput signal from the transponder 3 to the beacon 4 also compensatesfor the attenuation fluctuations of the UPLINK. Thus, the pathattenuations of the DOWN and UPLINK are compensated through the doublegain control of the transponder 3. The final outcome of this is that avehicle in a neighboring lane which is further away than the beacon'sown lane is no longer able to interfere. Therefore, a toll can bedebited for the desired lane without any interference and, thus,reliably.

We claim:
 1. A transponder for transmitting data to a fixed station viaa passive transponder method, the transponder including a receivingantenna and a transmitting antenna, the transponder comprising:a firstautomatically controllable amplifier connected in series with a secondautomatically controllable amplifier, the first and the secondamplifiers each being switchably coupled between the receiving antennaand the transmitting antenna, the first amplifier having a first outputsignal; a generator providing at least one predetermined setpoint value;a control amplifier coupled to the generator to receive the at least onepredetermined setpoint value, the control amplifier also being coupledto the first amplifier and generating a control value as a function ofthe first output signal of the first amplifier and the at least onepredetermined setpoint value; and wherein an amplification of the firstamplifier is controlled as a function of the control value, the controlvalue also being provided to the second amplifier.
 2. The transponder asrecited in claim 1, wherein the data is bidirectionally transmitted tothe fixed station, the fixed station including a beacon.
 3. Thetransponder as recited in claim 1, wherein the first and the secondamplifiers each have a substantially conforming control characteristic.4. The transponder as recited in claim 1, wherein the generator includesa memory unit for storing the at least one setpoint value.
 5. Thetransponder as recited in claim 1, wherein the first amplifier includesa first control input and the second amplifier includes a second controlinput, the first and second amplifiers each being controllable via therespective first and second control inputs.
 6. The transponder asrecited in claim 1, wherein the first and the second amplifiers eachhave a respective attenuation characteristic.
 7. The transponder asrecited in claim 6, wherein a second output signal of the secondamplifier has a path attenuation that quadratically increases as afunction of an amplitude of a first control input of the firstamplifier, the respective attenuation characteristics of the first andsecond amplifiers being controllable.
 8. The transponder as recited inclaim 1, wherein the second output is coupled to the transmittingantenna.
 9. The transponder as recited in claim 1, wherein the first andsecond amplifiers are mounted on a motor vehicle.
 10. The transponder asrecited in claim 1, wherein the receiving and transmitting antennascommunicate with a toll debiting system.
 11. The transponder as recitedin claim 1, wherein the receiving and transmitting antennas communicatewith a traffic control system.
 12. A transponder for transmitting datato a fixed station via a passive transponder method, the transponderhaving a receiving antenna and a transmitting antenna, the transpondercomprising:a first automatically controllable amplifier connected inseries with a second controllable amplifier, the first and the secondamplifiers each being coupled between the receiving antenna and thetransmitting antenna, the first amplifier having a first input and afirst output, the second amplifier having a second input and a secondoutput, the second output being coupled to the transmitting antenna; agenerator for providing at least one predetermined setpoint value; acontrol amplifier coupled to the generator to receive the at least onepredetermined setpoint value, the control amplifier also being coupledto the first input of the first amplifier and generating a control valueas a function of a first output signal of the first output and the atleast one predetermined setpoint value; and wherein the amplification ofthe first amplifier is controlled as a function of the control value,the control value also being provided to the second input of the secondamplifier.